* {
  margin: 0;
  padding: 0;
}

body {
  background-color: #ccc;
}

.box {
  margin: 0 auto;
  width: 400px;
  height: 300px;
  background-color: skyblue;
  position: relative;
  transition: background-image 0.2s ease-in-out;
  background-repeat: no-repeat;
  background-size: cover;
}

.left {
  cursor: pointer;
  font-size: 40px;
  font-weight: bold;
  position: absolute;
  height: 50px;
  width: 30px;
  top: 50%;
  left: 0;
  transform: translateY(-50%);
}

.right {
  cursor: pointer;
  font-size: 40px;
  font-weight: bold;
  position: absolute;
  height: 50px;
  width: 30px;
  top: 50%;
  right: 0;
  transform: translateY(-50%);
}

.box-1 {
  width: 400px;
  height: 100px;
  margin: 0 auto;
  display: flex;
  justify-content: center;
  align-items: center;
}

.circle {
  margin: 0 20px;
  width: 30px;
  height: 30px;
  border-radius: 50%;
  background-color: white;
  cursor: pointer;
}

.active {
  background-color: skyblue;
}

/* 作业打印格式 */
pre {
  margin-top: 100px;
}
